update.js

// I want to nullify segment_id for the model before saving if it was not changed;
// is it okay to mess with _dirtyAttributes ?

  actions: {
    updateParticipant: function() {
      if (!this.get('model._dirtyAttributes').contains('segment_id')) {
        this.get('model').set('segment_id', null);
      }
      return this.get('model').save().then(function() {
        return console.log("NOTIFY THE USER!");
      });
    }
  }

this works but I don't like it...

this.get('model').set('segment_id', false);

Assuming Ember Data, then you can use model.changedAttributes().

See http://emberjs.com/guides/models/working-with-records/#toc_modifying-attributes for more details.